(1) Field of the Invention
This invention relates to a transfer sheet and a process for the preparation thereof. More particularly, the invention relates to a transfer sheet for electrostatically transferring an electrically conductive or electrically semi-conductive toner in electrostatic photography or electrostatic printing, and a process for the preparation of such transfer sheet.
More specifically, the invention relates to a transfer sheet having a toner-receiving layer on which an electrically conductive or semi-conductive layer can be transferred at a very high transfer efficiency without broadening of contours of a toner image even under a high humidity condition and which is especially excellent in such properties as the feel and touch and the graphic property, and a process for the preparation of such transfer sheet.
(2) Description of the Prior Arts
As one of dry developers (toners) for developing electrostatic latent images formed by electrostatic photography or the like, a so-called electroconductive or semi-conductive magnetic toner capable of performing development without the aid of a particular carrier is known. As the toner of this type, there have heretofore been used toners formed by dispersing powder of a magnetic material such as triiron tetroxide, if necessary with a conducting agent such as carbon black, into a binder resin and molding the dispersion into granules. As means for improving the electric conductivity in these toners, there have ordinarily been adopted a method in which the amount of the conducting agent incorporated in the magnetic material-binder resin dispersion as increased and a method in which the conducting agent is embedded in the above-mentioned toner particles. By adopting these methods, toner particles are provided with such property that they can be magnetically attracted, and improved electric conductivity is imparted to surfaces of toner particles.
These magnetic toners have an advantage that sharp and clear toner images having a much reduced edge effect can be obtained according to the magnetic brush development method even without use of a magnetic carrier or the like. However, they have a defect that if toner images formed on photosensitive layers for electrostatic photography or electrostatic printing, such as photoconductive layers, are transferred onto copy papers, contours of the transferred images become obscure and no sharp images can be obtained.
More specifically, a toner image formed on a zinc oxide photosensitive layer for use in electrostatic photography or electrostatic printing has a good contrast and a sharp edge, but when this toner image is transferred onto untreated high quality paper which has heretofore been used broadly as the transfer sheet, as illustrated in Comparative Test 2 given hereinafter, not only extreme reduction of the density of the transferred image but also broadening of contours of the image take place, and the image becomes very obscure.
As means for eliminating this defect, Japanese Patent Application Laid-Open Specification No. 117435/75 (Japanese Patent Application No. 13929/74) proposes a method in which an electrically conductive or semi-conductive toner-receiving layer having a volume resistivity of at least 3.times.10.sup.13 .OMEGA.-cm is formed on at least one surface of a substrate of a transfer sheet, and it is taught that a medium such as a resin, a wax, an oil or fat or an insulating filler is applied to the surface of the substrate for forming such toner-receiving layer. It also is taught that an acrylic resin, a silicone resin, a vinyl acetate resin or an alkyd resin is preferred as the resin to be used for formation of such toner-receiving layer.
Indeed, a transfer sheet having a toner-receiving surface formed by using such resin is advantageous in that a toner image can be transferred without substantial broadening of contours under a relatively low humidity condition. However, this transfer sheet is still insufficient because under a high humidity condition broadening of contours of a toner image is caused at the transfer step and the transfer efficiency is relatively low. This degradation of transfer characteristics under a high humidity condition is especially conspicuous when such resin is applied in the form of an aqueous coating composition. Further, the toner-receiving layer of such resin tends to cause marked reduction of the feed and touch and graphic property of the transfer sheet, and when a pigment or extender pigment customarily used for improving these characteristics is incorporated in such toner-receiving layer, the above-mentioned humidity dependency of the transfer characteristics becomes more conspicuous.